1. Field of the Invention
The present invention relates generally to the generation of thermal scenes, and more specifically to an apparatus for generating high resolution thermal scenes, primarily used as test targets for the evaluation of near-to-far infrared electro-optical sensors.
2. Prior Art
There are numerous applications for a generator of dynamic, variable intensity, high resolution thermal scenes or patterns. Such a generator would find application in the biomedical field, as well as in secured communications. However, the principal current use for such a generator is for producing test targets for the evaluation of electro-optical sensors in the near to far infrared spectral range. Presently available thermal scene generators utilize resistively heated fibers, filaments or membranes that are typically addressed or powered serially in time. Their construction is usually very labor intensive or extremely process yield sensitive, and consequently, they are very expensive to fabricate. Furthermore, because of their serialized addressing and drive format, their data rates and associated "off target" driver electronics become, large, complex and expensive. An additional significant disadvantage of current thermal scene generators is their inability to operate over large thermal environments, particularly at reduced temperatures between approximately 70 degrees Kelvin and 250 degrees Kelvin, which is an operating requirement for most moderate to low background sensor systems. The prior art known to the applicant includes a number of U.S. Patents of varying degrees of relevance. These patents include the following:
U.S. Pat. No. 3,735,137 Bly PA1 U.S. Pat. No. 4,705,952 Lindmayer PA1 U.S. Pat. No. 4,760,267 Pistor PA1 U.S. Pat. No. 4,769,527 Hart et al PA1 U.S. Pat. No. 4,859,080 Titus et al PA1 U.S. Pat. No. 4,922,116 Grinberg et al PA1 U.S. Pat. No. 4,929,841 Chang PA1 U.S. Pat. No. 4,948,957 Rusche PA1 U.S. Pat. No. 4,967,089 Reilly et al PA1 U.S. Pat. No. 4,999,502 Midavaine PA1 U.S. Pat. No. 5,097,139 Foster
The most pertinent of the aforementioned patents may be summarized as follows:
U.S. Patent No. 4,967,089 to Reilly et al is directed to a pulsed optical source for transforming photons of a first wavelength into photoelectrons which are subsequently transformed into photons of a second wavelength. Referring to FIG. 1, there is shown a system wherein photons emitted from a source are converted to electrons by means of a photocathode, the electrons being directed from the photocathode to a multiplying and emitting means. The multiplying means includes a microchannel plate electron multiplier, the electron beam output from which is directed to a phosphor coating whose photon emission can be selected at any wavelength from ultraviolet to infrared, depending upon the particular phosphor selected. However, there is no disclosure of a pixel-by-pixel conversion between the input and output of the device.
U.S. Pat. No. 3,735,137 to Bly is directed to a screen panel for converting an optical image in the visible bandwidth to an infrared image. The screen panel is provided with conductive lattices on opposing sides of a dielectric substrate. The lattices define a matrix of individual cells which in effect define pixels of the resulting scene displayed on the panel. Overlaying the conductors is a resistive layer whose thermal output is responsive to changes in current flowing between the conductors and a conductor disposed within a hole centrally located in each of the pixel representative squares. On the opposing side of the panel a layer of photoconductive material is disposed over the lattice, such that a change in light impinging upon the photo-conductive material provides a proportional change in current through the resistive material to thereby provide the respective change in infrared radiation emitted therefrom.
U.S. Pat. Nos. 4,769,527 (Hart et al); 4,859,080 (Titus et al); and 4,922,116 (Grinberg et al) are directed to thermal image display systems. These references disclose devices are of interest in that they provide an infrared display wherein individual pixels are driven to radiate the infrared image.
U.S. Pat. No. 4,929,841 to Chang is directed to a dynamic infrared target. The target is formed by a photo-conductive film deposited on a transparent substrate wherein visible light photons which impinge upon the film through the substrate are absorbed, resulting in redistributing electrons from the valance band to the conductive band of the film to produce an infrared emission. An infrared image is produced by impinging the film with an image from a light projector.
U.S. Pat. No. 4,999,502 to Midavaine is directed to a system for generating an infrared image. As shown in the figures, a screen is mounted to a cathode ray tube for transforming an electron beam into infrared radiation. The screen is defined by a matrix of slabs, separated by a lattice of furrows to define thermal pixels.
In each of the aforementioned prior art patents relevant to the present invention, there is disclosed an infrared scene generator which suffers from one or more disadvantages relating to either extremely expensive construction, relatively low fabrication yield, complex off target drive electronics or the inability to operate over large thermal environments. There is therefore, still an unfilled need for a thermal scene generator which utilizes components and materials that can operate over a large thermal range, thereby obviating thermal background problems. There is also a need for thermal scene generators which are simple in construction and operation and are compatible with low cost production and utilization. Furthermore, there is a need for thermal scene generators which are implemented using parallel processing features so that data rates related to the time delay between visual or photon scene "IN" and thermal scene "OUT" can be extremely high.